Introduction

Global climate change is causing an increment in the frequency of extreme weather events (Stocker et al. 2014) that are recognized among the major drivers of current and future ecosystem dynamics (Franket al. 2015). The Mediterranean basin is one of the two main hot-spots of climate change (Giorgi 2006; Noce, Collalti & Santini 2017), showing increases in the inter annual variability and of extreme environmental conditions(Flaounas et al. 2013). In this region, the increasing risk of late frost events represents one of the major threats associated with the future global change (Zohner et al.2020). Indeed, increasing spring temperatures has been observed stimulating earlier leaf unfolding (Gordo & Sanz 2010; Allevatoet al. 2019), thus potentially exposing young leaves and shoots to spring frost damage (Augspurger 2013), especially at high elevation (Vitasse, Schneider, Rixen, Christen & Rebetez 2018). Depending on species, temperatures below −4°C can destroy the “fresh” leaves and shoots reducing − up to even blocking − the photosynthetic capacity of trees for several weeks. In this case, the resource requirements for new leaves formation, and trees life maintenance, must necessarily rely on the remobilization of carbon (C) reserves (Dittmar, Fricke & Elling 2006; D’Andrea et al. 2019). Moreover, severity, duration, and frequency of drought events have all been increasing in the last decades (Spinoni, Naumann, Vogt & Barbosa 2015). European beech (Fagus sylvatica L.), one of the most diffused native tree species in Europe, is known to be drought sensitive (Bolte et al. 2016). Hence, drought events can negatively affect physiological performance (Rezaieet al. 2018), carbon allocation (D’Andrea, Guidolotti, Scartazza, De Angelis & Matteucci 2020a), reproductive capacity (Nussbaumeret al. 2020), as well as the growth and competitive strength of the species (Peuke, Schraml, Hartung & Rennenberg 2002) which may all impact its future distribution (Noce et al. 2017).
Growth and non-structural carbohydrates (NSCs; i.e. sucrose, fructose, glucose and starch) dynamic are among the most strongly affected ecosystem processes by spring frost and summer drought (Li et al.2018). An increasing body of evidence has shown that NSCs dynamic is not a pure passive deposit and removal of C compounds, but it represents a key process actively controlled by plants to finely regulate C source-sink balance and to buffer the difference between C supply and demand at different timescales (Scartazza et al. 2001; Sala, Woodruff & Meinzer 2012; Carbone et al. 2013; Fatichi, Leuzinger & Körner 2014; Martínez-Vilalta et al. 2016; Moscatello et al. 2017; Collalti et al. 2020). Therefore, NSCs could play a crucial role in counteracting the negative effects of extreme weather events on beech forests, contributing to their resilience and survival (Scartazza, Moscatello, Matteucci, Battistelli & Brugnoli 2013; D’Andrea et al. 2019). However, despite the recognized importance of NSCs for plant productivity and resilience, little is known regarding their seasonal regulation and trade-off with growth and reproduction in forest trees (Merganičová et al. 2019; Tixier et al.2020). The stemwood plays a key role in the aforementioned mechanisms, because it, together with coarse roots, contains most of the NSCs of the tree plants (Barbaroux, Bréda & Dufrêne 2003; Richardson et al.2015)
In this work, we studied the effects of spring late frost and summer drought in a long-term research plot established in a Mediterranean beech forest (Collelongo, Abruzzi Region, Italy). The site is located in the large area of the Central-South Italy where in spring 2016, due to unusually warm preceding weeks, leaf unfolding occurred up to 15-20 days earlier than the normal average, followed by frost, that caused the complete loss of the newly grown leaves. Moreover, in 2017, a strong summer drought, due to a combination of drastic reduction of precipitation associated with high air temperature in late July and August, interested a huge area of the Mediterranean basin (Basciettoet al. , 2018; Nolè et al. , 2018; Allevato et al. , 2019; Rita et al. , 2019), including the Collelongo site. Notwithstanding these events were monitored through remote sensing techniques, in situ evaluations of their effects on ecosystem functionality are limited. Phenology, growth and stemwood NSCs dynamic in the Collelongo beech stand were investigated during 2016 (i.e. the year of the late frost event) and 2017 (i.e. the year of the summer drought event) and compared to the historical intra-annual data collected earlier at the site.
The objectives of the study were to: i ) quantify the magnitude of the effects of such extreme events on ecosystem functioning; ii ) verify the role of NSCs in mediating source-sink balance following the strong alteration of C activity; iii ) evaluate the interplay and trade-off between carbon allocation to canopy, stem and C reserves. The aim was to predict how these responses and regulation processes could contribute to the resilience of beech to extreme weather events associated with future global change.